command: This option lets you set the command that should be executed when Compose runs the image. Note that this will override the CMD instruction that we set in our applicationDockerfile. Here, we are running the application using the wait-for script, which will poll the db service on port 27017 to test whether or not the database service is ready. Once the readiness test succeeds, the script will execute the command we have set,/home/node/app/node_modules/.bin/nodemon app.js, to start the application withnodemon. This will ensure that any future changes we make to our code are reloaded without our having to restart the application.